Sliding member

ABSTRACT

Provided is a sliding member for a thrust bearing. The sliding member includes a back-metal layer and a sliding layer, and has a partially annular shape. The sliding layer includes a synthetic resin and has a sliding surface. In a center line region of the sliding layer, the sliding layer has a linear expansion coefficient KS in a direction parallel to a circumferential direction of the sliding member, a linear expansion coefficient KJ in a direction parallel to a radial direction of the sliding member, and a linear expansion coefficient KT in a direction perpendicular to the sliding surface, and the linear expansion coefficients KS, KJ, and KT satisfy the following relations (1) and (2): Relation (1): 1.1≤KS/KJ≤2; and Relation (2): 1.3≤KT/{(KS+KJ)/2}≤2.5.

TECHNICAL FIELD

The present invention relates to a sliding member for a thrust bearing,and specifically relates to a sliding member that has a partiallyannular shape and includes a back-metal layer and a sliding layerincluding a synthetic resin composition. The present invention alsorelates to a thrust bearing including the sliding member.

RELATED ART

A thrust bearing has been used as a thrust bearing for a rotating shaftof an exhaust turbine, a large-sized power generator or the like. Suchthrust bearing is configured that a plurality of bearing-pad-shapedsliding members having a partially annular shape are arranged in acircumferential direction to face a thrust collar surface of therotating shaft. In such a tilting pad thrust bearing, the slidingmembers having a partially annular shape are supported so that thesliding members can slightly swing by a pivot with respect to the thrustcollar surface of the shaft member. During steady operation of theexhaust turbine, the large-sized power generator or the like, alubricant flows between the thrust collar surface of the shaft memberand a sliding surface of the sliding member as the shaft member isrotated. At this time, the sliding member swings, and a gap between thesliding surface and the thrust collar surface of the shaft member isgradually reduced in a rotation direction. Thus, dynamic pressure isgenerated due to a wedge effect, and the lubricant forms a fluid film.The fluid film supports an axial load of the rotating shaft. In general,a swing center of a sliding member having a partially annular shape islocated in a center portion of a circumferential direction and a centerportion of a radial direction of the sliding member having the partiallyannular shape. A pressure distribution of the fluid film becomes maximumat the swing center of the sliding member having a partially annularshape (see JP 2015-94373A, paragraph [0020] and FIG. 2, for example).

As a sliding member of such a thrust bearing, a sliding member is knownin which a sliding layer including a resin composition is coated on aback-metal layer made of metal (see JP 2001-124062A, for example). Forexample, JP 10-204282A, JP 2016-079391A and JP 2013-194204A describe asliding layer including a resin composition in which fibrous particlessuch as glass fiber particles, carbon fiber particles or intermetalliccompound fiber particles are dispersed in a synthetic resin to enhance astrength of the sliding layer.

Furthermore, JP 2018-146060A describes a method of manufacturing a resincomposition sheet, including cooling the sheet in a forming mold andperiodically changing a drawing speed of a drawing roll.

SUMMARY OF THE INVENTION

During steady operation of the exhaust turbine, the large-sized powergenerator or the like, a fluid film of oil or the like is formed betweenthe shaft member and the sliding member to prevent direct contactbetween the surface of the shaft member and the sliding surface thesliding member. However, during operation, in particular when the shaftrotates with a high speed, a centrifugal force has a large influence onthe fluid film. In the vicinity of the swing center (in the vicinity ofa center portion of the circumferential direction and a center portionof the radial direction of the sliding member having a partially annularshape), the pressure of the fluid film is maximum and a resincomposition near the sliding surface is pressed by the pressure of thefluid film. Thus, the resin composition is elastically deformed in anouter diameter direction of the sliding member.

It was found that, in this case, if the resin composition of the slidinglayer is thermally expanded, due to frictional heat generated bysliding, in an almost isotropic manner in an in-plane direction of thesliding surface, damage such as a crack that extends in a substantiallycircumferential direction is highly likely to occur on the surface ofthe sliding layer, when the resin composition is elastically deformeddue to the pressure of the fluid film.

Furthermore, another problem has been found that, when an amount ofthermal expansion of the resin composition of the sliding layer isapproximately same between the in-plane direction of the sliding surfaceand a direction perpendicular to the sliding surface (thicknessdirection), shear failure is more likely to occur at an interfacebetween the back-metal layer made of metal and the sliding layer.

Thus, an object of the present invention is to overcome thedisadvantages of the conventional technique and provide a sliding memberthat is less likely to be subjected to damage such as a crack on asurface of a sliding layer and shear failure between the sliding layerand a back-metal layer, during the operation of a bearing device

According to an aspect of the present invention, provided is a slidingmember for a thrust bearing. The sliding member includes a back-metallayer and a sliding layer on the back-metal layer, and has a partiallyannular shape. The sliding layer includes a synthetic resin and has asliding surface. In a center line region of the sliding layer, thesliding layer has a linear expansion coefficient KS in a directionparallel to a circumferential direction of the sliding member, a linearexpansion coefficient KJ in a radial direction of the sliding member,and a linear expansion coefficient KT in a direction perpendicular tothe sliding surface, and KS, KJ, and KT satisfy the following relations(1) and (2):1.1≤KS/KJ≤2;  relation (1); and1.3≤KT/{(KS+KJ)/2}≤2.5  relation (2).

According to an embodiment of the present invention, KS and KJ of thesliding layer preferably satisfy the following relation (3):1.1≤KS/KJ≤1.7  relation (3).

According to an embodiment of the present invention, the synthetic resinpreferably includes one or more selected from polyether ether ketone,polyether ketone, polyether sulfone, polyamidimide, polyimide,polybenzimidazole, nylon, phenol, epoxy, polyacetal, polyphenylenesulfide, polyethylene, and polyetherimide.

According to an embodiment of the present invention, the sliding layerpreferably further includes 1 to 20 volume % of one or more solidlubricants selected from graphite, molybdenum disulfide, tungstendisulfide, boron nitride, and polytetrafluoroethylene.

According to an embodiment of the present invention, the sliding layerpreferably further includes 1 to 10 volume % of one or more fillersselected from CaF₂, CaCO₃, talc, mica, mullite, iron oxide, calciumphosphate, potassium titanate, and Mo₂C.

According to an embodiment of the present invention, the sliding layerpreferably further includes 1 to 35 volume % of one or more types offibrous particles selected from glass fiber particles, ceramic fiberparticles, carbon fiber particles, aramid fiber particles, acrylic fiberparticles, and polyvinyl alcohol fiber particles.

According to an embodiment of the present invention, the back-metallayer preferably has a porous metal portion on a surface which is aninterface between the back-metal layer and the sliding layer.

According to another aspect of the present invention, provided is athrust bearing including a plurality of the sliding members describedabove.

A configuration and advantages of the present invention are described indetail below with reference to the accompanying drawings. The drawingsillustrate embodiments merely for illustration purpose, and the presentinvention is not limited to the embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section of a sliding member according to anembodiment of the present invention.

FIG. 2 shows a cross section of a sliding member according to anotherembodiment of the present invention.

FIG. 3 is a schematic diagram of an embodiment of the sliding memberaccording to the present invention.

FIG. 4 is a schematic diagram showing an image of a molecular chain(bent structure) of a resin.

FIG. 5 is a view showing a flow of resin.

FIG. 6 shows a VI-VI cross section of a resin sheet in FIG. 5.

FIG. 7 is a plan view of a sliding surface of the sliding member forshowing a center line region.

DETAILED DESCRIPTION OF THE EMBODIMENT

FIG. 3 schematically shows an embodiment of a sliding member 1 having apartially annular shape according to the present invention. The slidingmember 1 has a flat shape, and a surface of the flat shape has a shapeobtained by cutting an annular ring (hereinafter referred to as“original annular ring”) along two lines extending in a radialdirection, that is, a partially annular shape. The partially annularshape preferably has a center angle of 25° to 60°, but the center angleof the partially annular shape is not limited to these angles. Thesliding member 1 is configured such that a sliding layer 3 is formed ona back-metal layer 2 having a flat and partially annular shape. Asurface of the sliding layer 3 having a partially annular shape is asliding surface 30.

Herein, a “circumferential direction” of the partially annular shape isa direction corresponding to a circumferential direction of the originalannular ring, and a “radial direction” of the partially annular shape isa direction corresponding to a radial direction of the original annularring. The circumferential direction of the partially annular shape isalso referred to as “S direction”, and the radial direction of thepartially annular shape is also referred to as “J direction”.Furthermore, a thickness direction of the partially annular shape, thatis, a direction perpendicular to the surface (sliding surface 30) of thepartially annular shape is referred to as “vertical direction” (Tdirection). Furthermore, an imaginary radial line that passes through acircumferential center of the partially annular shape is referred to as“center axis” or “center line”. When the sliding member 1 is used as abearing, a shaft member which is a counter member slides in the“circumferential direction”, and thus the “circumferential direction” isa sliding direction.

Next, a “center line region” of the sliding layer 3 is described. FIG. 7is a plan view of the sliding layer 3 of the sliding member 1 viewedfrom the sliding surface 30 side. The sliding surface 30 has the samepartially annular shape as the sliding member 1, and a center axis 34can be defined as described above. When a circumferential angle isdefined as an angle formed by two straight lines extending in the radialdirection, a region between two imaginary radial lines 35 separated fromeach other by a circumferential angle (θ1) of ±3° with respect to thecenter axis 34 (an angle formed by the radial lines 35, i.e., thecircumferential angle is 6°) is defined as “center line region 31”. Thatis, the “center line region 31” is a region surrounded by the twoimaginary radial lines 35 and an outer periphery and an inner peripheryof the sliding layer 3.

The “center line region 31” is not limited to the sliding surface 30,and includes a volume of a portion extending along the entire thicknessof the sliding layer 3.

FIG. 1 schematically shows a cross section of the sliding member 1according to the present invention. The sliding member 1 includes thesliding layer 3 including a synthetic resin 4 on the back-metal layer 2.The surface of the sliding layer 3 (on a side opposite to the back-metallayer) functions as the sliding surface 30. The cross section shown inFIG. 1 is a cross section of the sliding member 1 in a directionperpendicular to the sliding surface 30.

The synthetic resin 4 preferably includes one or more selected frompolyether ether ketone, polyether ketone, polyether sulfone,polyamidimide, polyimide, polybenzimidazole, nylon, phenol, epoxy,polyacetal, polyphenylene sulfide, polyethylene, and polyetherimide.

The sliding layer 3 may further include 1 to 20 volume % of one or moresolid lubricants selected from graphite, molybdenum disulfide, tungstendisulfide, boron nitride, and polytetrafluoroethylene. The solidlubricant preferably has an average grain size of 0.5 to 20 μm. Thesliding layer including the solid lubricant can have better slidingproperties. The sliding layer 3 may further include 1 to 10 volume % ofone or more fillers selected from CaF₂, CaCO₃, talc, mica, mullite, ironoxide, calcium phosphate, and Mo₂C (molybdenum carbide). The fillerpreferably has an average grain size of 0.1 to 10 μm. The sliding layerincluding the filler can have higher wear resistance.

The sliding layer 3 may further include 1 to 35 volume % of fibrousparticles dispersed in the synthetic resin 4. The fibrous particles arepreferably one or more types of fibrous particles selected from glassfiber particles, ceramic fiber particles, carbon fiber particles, aramidfiber particles, acrylic fiber particles, polyvinyl alcohol fiberparticles. The sliding layer including the fibrous particles can havehigher strength.

The fibrous particles may have an average grain size of 0.1 to 25 μm(the grain size of each of the fibrous particles is a diameter of aperfect circle having an area equal to an area of the fibrous particlemeasured in cross-sectional observation, i.e., “equivalent circlediameter”). When the sliding member is used in a bearing deviceconfigured such that a high load is applied to the sliding layer, thesliding layer preferably includes fibrous particles having an averagegrain size of not less than 0.1 μm and less than 5 μm and a major axislength of not more than 15 μm. If the sliding layer includes fibrousparticles having a major axis length of more than 15 μm, in some cases,a crack occurs in the large fibrous particles having a major axis lengthof more than 15 μm and exposed on the sliding surface, and the fibrousparticles fall into a gap between the sliding surface and a surface ofthe shaft member, resulting in damage to the sliding surface. On theother hand, in the case of the sliding layer including fibrous particleshaving a major axis length of not more than 15 μm, even when the fibrousparticles are exposed on the sliding surface, a crack is less likely tooccur in the fibrous particles.

The back-metal layer 2 may be made of an Fe alloy such as hypoeutectoidsteel or stainless steel, or a Cu alloy.

The sliding layer 3 preferably has a thickness (i.e., a distance in adirection perpendicular to the sliding surface 30 from the slidingsurface 30 to an interface 7 between the sliding layer 3 and theback-metal layer 2) of 0.5 to 6 mm.

In the center line region 31 of the sliding layer 3, the sliding layer 3has a linear expansion coefficient KS in the “circumferential direction”(S direction) of the sliding member 1, a linear expansion coefficient KJin the “radial direction” (J direction) of the sliding member 1, and alinear expansion coefficient KT in the “vertical direction” (Tdirection) perpendicular to the sliding surface 30, and KS, KJ, and KTof the sliding layer 3 satisfy the following relations (1) and (2):1.1≤KS/KJ≤2;  relation (1); and1.3≤KT/{(KS+KJ)/2}≤2.5  relation (2).

In other words, in the center line region 31 of the sliding layer 3, thelinear expansion coefficient KS of the sliding layer in the“circumferential direction” (S direction) of the sliding member is 1.1to 2 times the linear expansion coefficient KJ of the sliding layer inthe “radial direction” (J direction) of the sliding member, and thelinear expansion coefficient KT of the sliding layer in the “verticaldirection” (T direction) perpendicular to the sliding surface is 1.3 to2.5 times an average value of the linear expansion coefficient KS of thesliding layer in the “circumferential direction” (S direction) of thesliding member and the linear expansion coefficient KJ of the slidinglayer in the “radial direction” (J direction) of the sliding member.

The linear expansion coefficients KS, KJ, and KT of the sliding layer 3are each an average linear expansion coefficient at a temperature of 23°C. to 100° C.

The linear expansion coefficient KS in the “circumferential direction”(S direction) of the sliding member is preferably 1.1 to 1.7 times thelinear expansion coefficient KJ in the “radial direction” (J direction)of the sliding member (i.e., 1.1≤KS/KJ≤1.7).

It is known that synthetic resins have a large amount of thermalexpansion. A synthetic resin is composed of a large number of molecularchains (high polymers) by which a large number of resin molecules arelinked together, and the molecular chains of the resin are merelyconnected by the Van der Waals force and weakly bonded to each other.Thus, when the temperature increases, a gap (distance) between themolecular chains is greatly increased. On the other hand, an amount ofthermal expansion in a longitudinal direction of the molecular chains ofthe resin connected by a covalent bond is small. When an external forceapplied to the synthetic resin causes breakage, breakage mainly occursbetween the weakly bonded molecular chains of the resin, and themolecular chain itself is less likely to be broken.

A swing center of the sliding member 1 having a partially annular shapeis located in the vicinity of a center portion of the circumferentialdirection and a center portion of the radial direction of the slidingmember 1. During operation of a bearing device, high pressure of a fluidfilm on which a centrifugal force acts is applied to a portion of thesliding surface 30 of the sliding layer 3 near the swing center.Accordingly, an external force generated by the pressure of the fluidfilm is largest near the swing center, and the external force is mostapplied in the radial direction (i.e., center axis direction) due to aninfluence of the centrifugal force generated in the fluid film. Thus,the synthetic resin 4 of the sliding layer 3 near the center axis 34 iselastically deformed in the radial direction J. Furthermore, the shaftmember is rotated at a high speed, and this causes a temperature of thefluid film to increase, and accordingly, a temperature of the slidinglayer 3 of the sliding member 1 also becomes high. At this time, if anamount of thermal expansion of the synthetic resin 4 of the slidinglayer 3 in the radial direction J of the sliding member 1 having apartially annular shape is large near the center axis 34 of the slidinglayer 3, a large gap is generated between molecular chains (linearportions 42) of the synthetic resin, and the external force due to thepressure of the fluid film is more likely to cause breakage. Thus,damage such as a crack that extends in a substantially circumferentialdirection from the broken portion as a starting point may occur on thesurface of the sliding layer.

In the center line region 31 of the sliding layer 3 of the slidingmember 1 of the present invention, the linear expansion coefficient KSof the sliding layer in the “circumferential direction” (S direction) ofthe sliding member is 1.1 to 2 times the linear expansion coefficient KJof the sliding layer in the “radial direction” (J direction) of thesliding member, and the linear expansion coefficient KT of the slidinglayer in the “vertical direction” (T direction) perpendicular to thesliding surface 30 is 1.3 to 2.5 times the average value of the linearexpansion coefficient KS of the sliding layer in the “circumferentialdirection” (S direction) of the sliding member and the linear expansioncoefficient KJ of the sliding layer in the “radial direction” (Jdirection) of the sliding member. This leads to a large amount ofthermal expansion of the sliding layer 3 in the circumferentialdirection (S direction) and the vertical direction (T direction) andprevention of thermal expansion of the sliding layer 3 in the radialdirection (J direction). Thus, even when an external force in the centeraxis direction (radial direction, S direction) is applied from the fluidfilm to the sliding surface 30, a crack that extends in a substantiallycircumferential direction is less likely to occur on the sliding surface30.

If an amount of thermal expansion of the sliding layer 3 in an in-planedirection (the circumferential direction S and the radial direction J)of the sliding surface 30 is large, shearing stress is generated, due toa difference in the amount of thermal expansion, at the interfacebetween the sliding layer 3 and the back-metal layer 2 made of metal,and shear failure may occur between the sliding layer 3 and theback-metal layer 2.

In the sliding layer 3 of the sliding member 1 of the present invention,the linear expansion coefficient KT of the sliding layer in the“vertical direction” (T direction) perpendicular to the sliding surface30 is 1.3 to 2.5 times the average value of the linear expansioncoefficient KS of the sliding layer in the “circumferential direction”(S direction) of the sliding member and the linear expansion coefficientKJ of the sliding layer in the “radial direction” (J direction) of thesliding member. This leads to a large amount of thermal expansion of thesliding layer 3 in the “vertical direction” (T direction) and preventionof thermal expansion of the sliding layer 3 in a direction parallel tothe sliding surface 30. Thus, shear failure is less likely to occurbetween the sliding layer 3 and the back-metal layer 2.

An anisotropy of thermal expansion of the sliding layer 3 is presumablycaused by orientation of the molecular chains of the resin molecules. Asshown in FIG. 4, a molecular chain 41 of the resin molecule in thesynthetic resin 4 of the sliding layer 3 has a bent structure (bentcrystal) having a plurality of linear portions 42. In a longitudinaldirection L1 of the linear portion 42, thermal expansion is less likelyto occur. In a direction L2 orthogonal to the longitudinal direction ofthe linear portion 42, a gap is formed between the linear portions 42,and thus thermal expansion is more likely to occur. The anisotropy ofthermal expansion of the sliding layer 3 of the sliding member 1 of thepresent invention is presumably caused by the fact that, in the centerline region 31 of the sliding layer 3, a ratio of the molecular chains41 of the resin in the sliding layer 3 that are oriented in thelongitudinal direction L1 of the linear portion 42 is different betweenthe “radial direction” (J direction) side, the “circumferentialdirection” (S direction) side, and the “vertical direction” (Tdirection) side. The anisotropy of thermal expansion of the slidinglayer 3 is generated during manufacture of a resin composition sheet(described later).

A different configuration from above-described configuration of thepresent invention has the following problems.

-   -   If in the center line region 31 of the sliding layer 3, the        linear expansion coefficient KS of the sliding layer in the        “circumferential direction” (S direction) is larger than the        linear expansion coefficient KJ of the sliding layer in the        “radial direction” (J direction) but is less than 1.1 times the        linear expansion coefficient KJ, the effect of preventing        thermal expansion of the sliding layer in the “radial direction”        (J direction) is insufficient, and a crack is more likely to        occur on the sliding surface.    -   If in the center line region 31 of the sliding layer 3, the        linear expansion coefficient KS of the sliding layer in the        “circumferential direction” (S direction) exceeds 2 times the        linear expansion coefficient KJ of the sliding layer in the        “radial direction” (J direction), due to an excessively large        amount of thermal expansion of the sliding layer in the        “circumferential direction” (S direction), a crack that extends        in a substantially radial direction may occur on the sliding        surface.    -   If in the center line region 31 of the sliding layer 3, the        linear expansion coefficient KT in the “vertical direction” (T        direction) perpendicular to the sliding surface is larger than        the average value of the linear expansion coefficient KS in the        “circumferential direction” (S direction) of the sliding member        and the linear expansion coefficient KJ in the “radial        direction” (J direction) of the sliding member but is less than        1.3 times the average value of the linear expansion coefficient        KS and the linear expansion coefficient KJ, the effect of        preventing thermal expansion of the sliding layer in a direction        parallel to the sliding surface is insufficient, and thus a        crack is more likely to occur on the sliding surface, and shear        failure is more likely to occur between the sliding layer 3 and        the back-metal layer 2. Furthermore, if in the sliding layer 3,        the linear expansion coefficient KT in the “vertical direction”        (T direction) perpendicular to the sliding surface exceeds 2.5        times the average value of the linear expansion coefficient KS        in the “circumferential direction” (S direction) of the sliding        member and the linear expansion coefficient KJ in the “radial        direction” (J direction) of the sliding member, due to an        excessively large amount of thermal expansion of the sliding        layer in the “vertical direction” (T direction), a crack may        occur inside the sliding layer.    -   Unlike the configuration of the present invention, in a case of        a sliding member in which isotropic thermal expansion occurs        throughout a sliding layer, during operation of a bearing        device, a resin composition near a surface of the sliding layer        of the sliding member is thermally expanded in the same manner        in the circumferential direction and in the radial direction,        and during elastic deformation in the radial direction due to an        external force from a fluid film, a crack occurs between        molecular chains of a resin of the sliding layer. Thus, damage        such as a crack is more likely to occur on the surface of the        sliding layer, and due to a difference in the amount of thermal        expansion between the sliding layer and a back-metal layer,        damage such as shear failure is more likely to occur at an        interface between the sliding layer and the back-metal layer.

The back-metal layer 2 may have a porous metal portion 5 at theinterface between the back-metal layer 2 and the sliding layer 3. FIG. 2schematically shows a circumferential cross section of an example of thesliding member 1 including the back-metal layer 2 having the porousmetal portion 5. The porous metal portion 5 provided on the surface ofthe back-metal layer 2 can improve bonding strength between the slidinglayer and the back-metal layer. This is due to an anchor effect byimpregnating into pores of the porous metal portion with the compositionconstituting the sliding layer. The anchor effect can enhance a bondingforce between the back-metal layer and the sliding layer.

The porous metal portion can be formed by sintering a metal powder madeof Cu, a Cu alloy, Fe, an Fe alloy or the like on a surface of a metalplate or strip or the like. The porous metal portion may have a porosityof approximately 20 to 60%. The porous metal portion may have athickness of approximately 50 to 500 μm. In this case, the sliding layercoated on a surface of the porous metal portion may have a thickness ofapproximately 0.5 to 6 mm. The dimensions described above are merelyexamples. The present invention is not limited to the above values, andthe dimensions may be changed to other dimensions.

The sliding member 1 may be used, for example, in a thrust bearing. Forexample, this bearing includes a housing having an annular recessedportion. A plurality of the sliding members are arranged in thecircumferential direction in the annular recessed portion, and thesliding members support a thrust collar surface of a counter shaft whichis a shaft member. The partially annular shape (curvature, size, and thelike) of the sliding member is designed to match the annular recessedportion and the shaft member. However, the sliding member may also beused in a bearing having a different configuration or for other slidingapplications.

The present invention also encompasses a thrust bearing including aplurality of the sliding members.

The above sliding member is described in detail below referring to amanufacturing process.

(1) Preparation of Synthetic Resin Raw Material Particles

A raw material of the synthetic resin may be one or more selected frompolyether ether ketone, polyether ketone, polyether sulfone,polyamidimide, polyimide, polybenzimidazole, nylon, phenol, epoxy,polyacetal, polyphenylene sulfide, polyethylene, and polyetherimide.Optionally, fibrous particles, a solid lubricant, a filler, or the likemay be dispersed in the synthetic resin.

(2) Manufacture of Synthetic Resin Sheet

A synthetic resin sheet is produced from the above raw material and thelike with use of a melt-kneading machine, a supplying mold, a sheetforming mold, and a cooling roll.

“Melt-Kneading Machine”

The synthetic resin raw material particles and raw materials of otheroptional materials (fibrous particles, solid lubricant, filler, and thelike) are mixed while being heated at a temperature of 230° C. to 390°C. with use of the melt-kneading machine to produce a resin compositionin a molten state. The synthetic resin raw material particles include aplurality of resin molecules having a structure in which a molecularchain is bent to have a plurality of linear portions. The resinmolecules entangled with each other are disentangled by themelt-kneading process. The resin composition is extruded under constantpressure from the melt-kneading machine.

“Supplying Mold”

A certain amount of resin composition extruded from the melt-kneadingmachine is constantly supplied to the sheet forming mold via thesupplying mold. The supplying mold includes a heating heater for heatingthe resin composition passing through the supplying mold at atemperature of 385° C. to 400° C. to maintain the resin composition in amolten state.

“Sheet Forming Mold”

The resin composition is formed into a sheet shape by the sheet formingmold. The resin composition in a molten state supplied from thesupplying mold to the sheet forming mold is formed into a sheet shape,and is gradually cooled naturally while being moved toward an outletside in the sheet forming mold to form a sheet in a semi-molten state.

“Cooling Roll”

The resin composition sheet in a semi-molten state is drawn from the“sheet forming mold” while being continuously brought into contact withthe cooling roll to be cooled. The cooling roll includes at least a pairof rolls (upper roll and lower roll) that move the resin compositionsheet while pressing the resin composition sheet from both sides, i.e.,an upper surface side and a lower surface side. After being drawn fromthe cooling roll, the resin composition sheet in a semi-molten statebecomes a sheet in a completely solid state. A temperature of thecooling roll can be controlled by an electric heater incorporated in theroll. Furthermore, the cooling roll can be rotationally driven by beingcontrolled by an electric motor. The resin composition sheet has athickness, for example, of 1 to 7 mm. The resin composition sheet in asolid state is cut into a size corresponding to that of a back metalused at a later-described coating step.

(3) Back Metal

The back-metal layer may be a metal plate made of an Fe alloy such ashypoeutectoid steel or stainless steel, Cu, a Cu alloy, or the like. Aporous metal portion may be formed on a surface of the back-metal layer,i.e., an interface between the back-metal layer and the sliding layer.In that case, the porous metal portion may have the same composition asthe back-metal layer. Alternatively, the porous metal portion may have adifferent composition from the back-metal layer or may be made of adifferent material from the back-metal layer.

(4) Coating and Forming Step

The resin composition sheet is bonded to one surface of the back-metallayer or the porous metal portion of the back metal. At that time, theresin composition sheet is placed so that the direction in which theresin composition sheet has been drawn at the sheet forming step isparallel to a center axis of a partially annular shape of an endproduct. Subsequently, the composition is formed by pressure pressinginto a shape for use, for example, a partially annular shape. Then, thesliding layer and the back metal are processed or cut so that thecomposition has a uniform thickness.

Next, a method of controlling the anisotropy of the linear expansioncoefficient is described. The anisotropy of the linear expansioncoefficient is controlled by controlling a rotational speed of thecooling roll in the process of manufacturing a resin composition sheet.Specifically, the rotational speed of the cooling roll is set so that aratio V₂/V₁ is 0.8 to 0.9, where V₁ represents a speed at which thesheet in a semi-molten state is extruded from the sheet forming mold,and V₂ represents a speed of the sheet in a completely solidified statedrawn from the cooling roll. The ratio of 0.8 to 0.9 is a ratio betweenv₁ and v₂ (v₂/v₁=V₂/V₁=0.8 to 0.9), where v₁ represents a volume perunit time of the resin composition sheet extruded from the sheet formingmold by pressure from the supplying mold and supplied to the coolingroll, and v₂ represents a volume per unit time of the resin compositionsheet drawn from the cooling roll. Due to a difference between thevolume v₁ of the resin composition sheet supplied to the cooling rolland the volume v₂ of the resin composition sheet drawn from the coolingroll, a resin sump 15 of the semi-molten resin composition is formed atan inlet of the upper cooling roll.

The resin sheet in a semi-molten state is solidified while being broughtinto contact with the cooling roll to be cooled. The speed of thecooling roll is set to be lower than the speed at which the resincomposition in a semi-molten state is extruded from the mold. Thus, theresin composition in a semi-molten state that has not been completelysolidified tends to be accumulated (hereinafter referred to as “resinsump”) at the inlet of the upper cooling roll. FIG. 5 schematicallyshows this state. A resin composition sheet 20 is extruded in adirection (extrusion direction 10) from the right side toward the leftside of FIG. 5. An arrow 11 indicates a flow of semi-molten resincomposition. The semi-molten resin composition 11 flowing from a sheetforming mold 12 (from the right side of FIG. 5) forms a certain amountof resin sump 15 on an inlet side of an upper cooling roll 13. The resincomposition 11 in a semi-molten state (hereinafter referred to as“semi-molten resin composition”) that forms the resin sump 15 is pushedinto the resin composition sheet while being rotated in the samedirection as the extrusion direction and accumulated. A part of thesemi-molten resin composition 11 pushed into the resin composition sheetis spread and flows toward both ends in a width direction of the resincomposition sheet and starts to be solidified (FIG. 6). It has beenfound that when the molten resin flows, the longitudinal directions ofthe linear portions of the molecular chains of the resin are more likelyto be oriented in the flow direction. Thus, the longitudinal directionsof the linear portions of the molecular chains of the resin are morelikely to be oriented also in the width direction of the resincomposition sheet (direction perpendicular to the extrusion direction10).

In a conventional technique, the speed of the cooling roll is set to thesame speed as the speed at which the resin composition in a semi-moltenstate is extruded from the mold. In this case, the semi-molten resincomposition flowing from the sheet forming mold constantly flows in asingle direction toward an outlet side of the cooling roll withoutforming a resin sump on the inlet side of the cooling roll. Thus, thelongitudinal directions of the linear portions of the molecular chainsof the resin are mainly oriented in the extrusion direction of the resincomposition sheet and are less likely to be oriented in the widthdirection of the resin composition sheet.

Furthermore, JP 2018-146060A discloses that a resin composition sheet ismanufactured by performing cooling in a forming mold and periodicallychanging a drawing speed of a drawing roll. When a resin compositionsheet is manufactured in this manner, the longitudinal directions oflinear portions of molecular chains of the resin are more likely to bemainly oriented in a thickness direction of the resin composition sheet.

Next, a method of measuring the linear expansion coefficient of thesliding layer is described. From the sliding layer, arectangular-parallelepiped-shaped specimen having a size of 4 mm×5 mm(measurement direction)×10 mm is produced to measure, in the center lineregion 31 of the sliding layer 3, the linear expansion coefficient KS ofthe sliding layer in a direction parallel to the circumferentialdirection and the linear expansion coefficient KJ of the sliding layerin a direction parallel to the radial direction. Furthermore, from thesliding layer, a rectangular-parallelepiped-shaped specimen having asize of 5 mm×5 mm×4 mm (measurement direction) is produced to measure,in the center line region 31 of the sliding layer 3, the linearexpansion coefficient KT of the sliding layer in a directionperpendicular to the sliding surface. Then, in the specimens, the linearexpansion coefficients KS, KJ, and KT can be measured under conditionsshown in Table 1 with use of a thermal expansion measuring device(TMA/SS7100: manufactured by SII). These linear expansion coefficientsare an average linear expansion coefficient in a test temperature range.

The specimens are obtained at arbitrary positions in the center lineregion 31.

TABLE 1 Test load (compressive load) 4 kPa Temperature increasing rate5° C./minute Measurement atmosphere Nitrogen (100 ml/minute) Testtemperature range 23 to 100° C.

EXAMPLES

Examples 1 to 6 of the sliding member including the back-metal layer andthe sliding layer according to the present invention and ComparativeExamples 11 to 14 were produced in the following manner. Table 2 showscomposition of the sliding layer of the sliding members of Examples andComparative Examples.

TABLE 2 Sliding test results Condition 1 Condition 2 Linear expansionPres- Presence Pres- Presence Composition (volume %) Coefficient KT/ence of shear ence of shear Ceramic Carbon Graph- (×10⁻⁵/° C.) ((KS + offailure at of failure at Sample PEEK PEK fibers fibers ite PTFE CaF2 KSKJ KT KS/KJ KJ)/2) cracks interface cracks interface Ex- 1 100 6.8 3.66.8 1.9 1.3 Not Not Present Not amples present present present 2 100 4.93.3 7.9 1.5 1.9 Not Not Not Not present present present present 3 1003.6 3.2 8.6 1.1 2.5 Not Not Not Not present present present present 4100 5 3.6 7.7 1.4 1.8 Not Not Not Not present present present present 578 20 2 4.6 2.8 6.3 1.6 1.7 Not Not Not Not present present presentpresent 6 78 15 5 2 4.3 2.9 6.4 1.5 1.8 Not Not Not Not present presentpresent present Com- 11 100 7.7 3.7 6.3 2.1 1.1 Present Not — — parativepresent Ex- 12 100 3.3 3.3 8.7 1.0 2.6 Present Not — — amples present 1380 15 5 2 2.9 4.3 6.4 0.7 1.8 Present Not — — present 14 78 20 2 4.7 7.73.8 0.6 0.6 Present Present — —

In Examples 1 to 6 and Comparative Examples 11 to 14, PEEK (polyetherether ketone) particles or PEK (polyether ketone) particles were used asa raw material of the synthetic resin. In Example 6 and ComparativeExample 13, the raw material of the synthetic resin included ceramicfibers. As the ceramic fibers, fibrous particles of potassium titanatehaving an average grain size of approximately 5 μm were used. In Example5 and Comparative Example 14, the raw material of the synthetic resinincluded carbon fibers. As the carbon fibers, fibrous particles havingan average grain size of 5 μm were used.

In Examples 5 and 6 and Comparative Examples 13 and 14, the raw materialof the synthetic resin included a solid lubricant (graphite, PTFE), andraw material particles of the solid lubricant had an average grain sizeof 10 μm. In Example 6 and Comparative Example 13, the raw material ofthe synthetic resin included a filler (CaF₂), and raw material particlesof the filler had an average grain size of 10 μm.

The above raw materials were weighed at a composition ratio shown inTable 2, and the compositions were pelleted in advance. The pellets wereinserted into a melt-kneading machine in which a heating temperature wasset at 350 to 390° C., and the pellets were sequentially passed througha supplying mold, a sheet forming mold, and a cooling roll to produce aresin composition sheet. A rotational speed of the cooling roll was setso that a ratio V₂/V₁ was 0.90 in Example 1, 0.85 in Examples 2 and 4 to6, and 0.80 in Example 3 to produce a resin composition sheet, where V₁represents a speed at which the sheet in a semi-molten state wasextruded from the sheet forming mold, and V₂ represents a speed of thesheet in a completely solidified state drawn from the cooling roll. Therotational speed of the cooling roll was set so that the ratio V₂/V₁ was1 in Comparative Example 11, 0.75 in Comparative Example 12, and 0.85 inComparative Example 13 to produce a resin composition sheet. InComparative Example 14, a resin composition sheet was produced by usingthe method described in JP 2018-146059A.

Next, the resin composition sheet was coated on one surface of aback-metal layer made of an Fe alloy, and was then processed into apartially annular shape. Subsequently, cutting processing was performedso that the composition on the back-metal layer had a predeterminedthickness. In Examples 1 to 5 and Comparative Examples 11 to 14, theback-metal layer was made of an Fe alloy. In Example 6, the back-metallayer had a porous sintered portion made of a Cu alloy on the surface ofthe portion made of an Fe alloy. For the sliding members of Examples 1to 6 and Comparative Examples 11, 12, and 14, an extrusion direction ofthe resin composition sheet at the sheet forming step was set to beparallel to a center axis direction of the partially annular shape. Forthe sliding member of Comparative Example 13, the extrusion direction ofthe resin composition sheet at the sheet forming step was set to beperpendicular to the center axis direction of the partially annularshape.

In the sliding members produced in Examples 1 to 6 and ComparativeExamples 11 to 14, the sliding layer had a thickness of 5 mm, and theback-metal layer had a thickness of 10 mm.

In the sliding members produced in Examples and Comparative Examples, inthe center line region of the sliding layer, the linear expansioncoefficient KS of the sliding layer in the circumferential direction,the linear expansion coefficient KJ of the sliding layer in the radialdirection, and the linear expansion coefficient KT of the sliding layerin the vertical direction were measured by the measurement methoddescribed above. These linear expansion coefficients were an averagelinear expansion coefficient at 23° C. to 100° C. In Table 2, withregard to the measurement results of Examples and Comparative Examples,a column “KS” indicates the linear expansion coefficient KS of thesliding layer in the circumferential direction, a column “KJ” indicatesthe linear expansion coefficient KJ of the sliding layer in the radialdirection, and a column “KT” indicates the linear expansion coefficientKT of the sliding layer in the vertical direction.

Furthermore, with regard to the measurement results of Examples andComparative Examples, a column “KS/KJ” in Table 2 indicates a ratio(KS/KJ) between the linear expansion coefficient KS of the sliding layerin the circumferential direction and the linear expansion coefficient KJof the sliding layer in the radial direction, and a column“KT/((KS+KJ)/2)” in Table 2 indicates a ratio (KT/(KS+KJ)/2) between thelinear expansion coefficient KT of the sliding layer in the verticaldirection and an average value of the linear expansion coefficient KJ ofthe sliding layer in the radial direction and the linear expansioncoefficient KS of the sliding layer in the circumferential direction.

The plurality of sliding members formed into a partially annular shapewere combined to form a thrust bearing, and the thrust bearing wassubjected to a sliding test under two conditions shown in Table 3. Underconditions 2, a rotational speed of the shaft member was higher thanunder conditions 1. These conditions simulated a sliding state duringnormal operation of a bearing device. In Examples and ComparativeExamples, a plurality of portions on a surface of the sliding layer inthe center line region after the sliding test were measured with use ofa roughness measuring device and evaluated for the presence of cracks.In a column “Presence of cracks” in Table 2, “Present” indicates that acrack having a depth of not less than 5 μm was observed on the surfaceof the sliding layer, and “Not present” indicates that no such crack wasobserved. Furthermore, a specimen of the sliding member after thesliding test was cut in a direction parallel to the center axis of thesliding member and perpendicular to the sliding surface, and thespecimen was observed for the presence of “shear failure” at aninterface between the sliding layer and the back metal with use of anoptical microscope. In a column “Presence of shear failure at interface”in Table 2, “Present” indicates that “shear failure” was observed at theinterface, and “Not present” indicates that no “shear failure” wasobserved at the interface.

TABLE 3 Conditions 1 Conditions 2 Testing device thrust tester thrusttester Load 30 MPa 30 MPa Rotational speed 3000 r/minute 6000 r/minuteOperation time 24 hours 24 hours Oil VG46 VG46 Oil supply amount duringoperation 50 L/minute 50 L/minute Oil supply temperature 50° C. 50° C.Counter shaft SUJ2 SUJ2 Shaft roughness 0.2 Ra 0.2 Ra

As shown in the results in Table 2, Examples showed no crack on thesurface of the sliding layer or no shear failure at the interface wasobserved after the sliding test under conditions 1. As described above,this is presumably because in the center line region of the slidinglayer, the sliding layer had the anisotropy of thermal expansion thatsatisfied the above relations (1) and (2) in the circumferentialdirection, the radial direction, and the vertical direction.Furthermore, in Examples 2 to 6 in which the sliding layer had theanisotropy of thermal expansion that satisfied the above relation (3),no crack was observed on the surface of the sliding layer after thesliding test even under conditions 2 under which higher pressure wasapplied to the sliding layer.

On the other hand, in Comparative Example 11, the ratio (KS/KJ) betweenthe linear expansion coefficient KS of the sliding layer in thecircumferential direction and the linear expansion coefficient KJ of thesliding layer in the radial direction exceeded 2. Presumably due to anexcessively large amount of thermal expansion of the sliding layer inthe circumferential direction, a crack occurred on the sliding surface.

On the other hand, in Comparative Example 12, the ratio (KS/KJ) betweenthe linear expansion coefficient KS of the sliding layer in thecircumferential direction and the linear expansion coefficient KJ of thesliding layer in the radial direction was less than 1.1. Presumably dueto an insufficient effect of preventing thermal expansion of the slidinglayer in the radial direction, a crack occurred on the sliding surface.Furthermore, in Comparative Example 12, since the ratio (KT/(KS+KJ)/2)between the linear expansion coefficient KT of the sliding layer in thevertical direction and the average value of the linear expansioncoefficient KJ of the sliding layer in the radial direction and thelinear expansion coefficient KS of the sliding layer in thecircumferential direction exceeded 2.5 times, a crack also occurredinside the sliding layer.

In Comparative Example 13, presumably since the ratio (KS/KJ) betweenthe linear expansion coefficient KS of the sliding layer in thecircumferential direction and the linear expansion coefficient KJ of thesliding layer in the radial direction was less than 1.1, a crackoccurred on the sliding layer.

In Comparative Example 14, since the ratio (KS/KJ) between the linearexpansion coefficient KS of the sliding layer in the circumferentialdirection and the linear expansion coefficient KJ of the sliding layerin the radial direction was less than 1.1, a crack occurred on thesliding layer. Furthermore, the ratio (KT/(KS+KJ)/2) between the linearexpansion coefficient KT of the sliding layer in the vertical directionand the average value of the linear expansion coefficient KJ of thesliding layer in the radial direction and the linear expansioncoefficient KS of the sliding layer in the circumferential direction wasless than 1.3. Presumably due to an insufficient effect of preventingthermal expansion of the sliding layer in a direction parallel to thesliding surface, shear failure occurred at the interface between thesliding layer and the back metal.

I claim:
 1. A sliding member for a thrust bearing, the sliding membercomprising: a back-metal layer; and a sliding layer on the back-metallayer, wherein the sliding member has a partially annular shape, whereinthe sliding layer includes a synthetic resin and has a sliding surface,and wherein, in a center line region of the sliding layer, the slidinglayer has a linear expansion coefficient KS in a direction parallel to acircumferential direction of the sliding member, a linear expansioncoefficient KJ in a direction parallel to a radial direction of thesliding member, and a linear expansion coefficient KT in a directionperpendicular to the sliding surface, and the linear expansioncoefficients KS, KJ and KT satisfy the following relations (1) and (2):1.1≤KS/KJ≤2;  relation (1); and1.3≤KT/{(KS+KJ)/2}≤2.5  relation (2).
 2. The sliding member according toclaim 1, wherein the linear expansion coefficients KS and KJ of thesliding layer satisfy the following relation (3):1.1≤KS/KJ≤1.7  relations (3).
 3. The sliding member according to claim1, wherein the synthetic resin includes one or more selected frompolyether ether ketone, polyether ketone, polyether sulfone,polyamidimide, polyimide, polybenzimidazole, nylon, phenol, epoxy,polyacetal, polyphenylene sulfide, polyethylene, and polyetherimide. 4.The sliding member according to claim 1, wherein the sliding layerfurther includes 1 to 20 volume % of one or more solid lubricantsselected from graphite, molybdenum disulfide, tungsten disulfide, boronnitride, and polytetrafluoroethylene.
 5. The sliding member according toclaim 1, wherein the sliding layer further includes 1 to 10 volume % ofone or more fillers selected from CaF₂, CaCO₃, talc, mica, mullite, ironoxide, calcium phosphate, potassium titanate, and Mo₂C.
 6. The slidingmember according to claim 1, wherein the sliding layer further includes1 to 35 volume % of one or more types of fibrous particles selected fromglass fiber particles, ceramic fiber particles, carbon fiber particles,aramid fiber particles, acrylic fiber particles, and polyvinyl alcoholfiber particles.
 7. The sliding member according to claim 1, wherein thesliding layer further includes: 1 to 20 volume % of one or more solidlubricants selected from graphite, molybdenum disulfide, tungstendisulfide, boron nitride, and polytetrafluoroethylene; and 1 to 10volume % of one or more fillers selected from CaF₂, CaCO₃, talc, mica,mullite, iron oxide, calcium phosphate, potassium titanate, and Mo₂C. 8.The sliding member according to claim 1, wherein the sliding layerfurther includes: 1 to 20 volume % of one or more solid lubricantsselected from graphite, molybdenum disulfide, tungsten disulfide, boronnitride, and polytetrafluoroethylene; and 1 to 35 volume % of one ormore types of fibrous particles selected from glass fiber particles,ceramic fiber particles, carbon fiber particles, aramid fiber particles,acrylic fiber particles, and polyvinyl alcohol fiber particles.
 9. Thesliding member according to claim 1, wherein the sliding layer furtherincludes: 1 to 20 volume % of one or more solid lubricants selected fromgraphite, molybdenum disulfide, tungsten disulfide, boron nitride, andpolytetrafluoroethylene; 1 to 10 volume % of one or more fillersselected from CaF₂, CaCO₃, talc, mica, mullite, iron oxide, calciumphosphate, potassium titanate, and Mo₂C; and 1 to 35 volume % of one ormore types of fibrous particles selected from glass fiber particles,ceramic fiber particles, carbon fiber particles, aramid fiber particles,acrylic fiber particles, and polyvinyl alcohol fiber particles.
 10. Thesliding member according to claim 1, wherein the back-metal layerincludes a porous metal portion as an interface between the back-metallayer and the sliding layer.
 11. A thrust bearing comprising a pluralityof the sliding members according to claim 1.